Method and apparatus for sealing tubes



June 2, 1964 c. c. STONE METHOD AND APPARATUS FOR SEALING TUBES 3Sheets-Sheet 1 Filed June 27, 1962 INVENTOR. Cecil C. Stone June 2, 1964c. c. STONE 3,135,356

METHOD AND APPARATUS FOR SEALING TUBES Filed June 27, 1962 3Sheets-Sheet 2 INVENTOR.

3 Sheets-Sheet 3 C. C. STONE mix 6. Stone flitorfigy METHOD ANDAPPARATUS FOR SEALING TUBES June 2, 1964 Filed June 27, 1962 UnitedStates Patent 3,135,856 METHOD AND APPARATUS FOR SEALING TUBES Cecil C.Stone, Downers Grove, Ill., assignor to the United States of America asrepresented by the United States Atomic Energy Commission Filed June 27,1962, Ser. No. 205,808 4 Ciaiins. (Cl. 219--124) The invention relatesto a novel method and apparatus for sealing tubes in tube sheets and,more particularly, to the fabrication of heat exchangers where a highdegree of freedom from leaks is required.

There is diificulty in making good sealing welds between tubes and atube sheet. If the weld is sufiicient for a good seal, the heat due toan arc weld may burn a tube and perforate it if his thin-walled. This isparticularly disadvantageous in a heat exchanger for a nuclear reactoroperating with an aqueous solution and liquid alkali metal, for thereaction between these liquids is of dangerous violence in the event ofa leak.

It is known to arc-weld tubes to a sheet, for example, when thetubes arevertical and a self-consuming electrode rotating about the tubes isused. There is great danger of burning the tube particularly iftheelectrode is self-consuming. In this case, the arc is frequentlydirected against the tube, rather than the weld metal, in such a way asto burn the tube. Even if the tube is not burned, such limited controlof the weld metal may be exerted as to produce a weld of limitedstrength and scaling properties.

It is an object of the present invention to provide a method andapparatus for producing an improved weld between a tube and a tubesheet.

A further object is to provide a method and apparatus for producing animproved arc weld in which the burning of parts being welded is avoidedand the weld metal is controlled so as to increase the strength andsealing properties of the metals.

Another object is to produce a weld of the above qualities between atube and a tube sheet by a method and apparatus employing a consumableelectrode Other objects will appear as the description proceeds.

All the foregoing objects are attained by my discovery that welds of anextremely high degree of reliability -may be made between tubes and atube sheet by rotating cupies or which tube is being welded. Moreover,my

method and apparatus are simple and flexible, requiring onlycomparatively inexpensive modifications of commercially obtainablewelding equipment, and permitting easy adjustments as the work proceeds.

Reference is now made to the drawings,

FIG. 1 of which is a front view of the apparatus for carrying out themethod of the invention.

FIG. 2 is a sectional side view of a portion of the same apparatus.

FIG. 3 is. a partly broken away top view of part of the same portion.

FIG. 4 is a sectional view of the same portion sectioned at right anglesto the View of FIG. 2.

Patented June 2, 1964 FIG. 5 is another sectional view of the sameportion sectioned somewhat closer to the face of the apparatus than theview of FIG. 4.

FIG. 6 is a third sectional View of the same portion sectioned stillcloser to'the face than the view of FIG. 5.

FIG. 7 is a broken-away, partly sectional view of the portion of FIG. 3.

FIG. 8 is another broken-away, partly sectional view of the sameportion.

FIG. 9 is a broken away, partly sectional view of the tube, tube sheetand weld therebetween, produced by the method of the invention.

As shown in FIGS.]1 and 2, an arc-welding gun 9 has a pistol grip 10 anda nozzle 11, directed at a tube sheet 12. The tube sheet has openingsreceiving a plurality of tubes 13 arranged in a geometric pattern, someof which are shown in detail, and others indicated schematically bysmall crosses. One of the plurality, tube 13a near the periphery of tubesheet 12, is shown in four positions it occupies as the method of theinvention is carried out. It willbe noted that at the top position, orat 12 oclock if tube sheet 12 is considered an imaginary clock dial,tube 13a has no weld around it. At the right, or at 3 oclock on the sameimaginary dial tube 13a is surrounded by a weld 14 beginning at about 11oclock if the boundary between the tube and the tube sheet is consideredto be a smaller imaginary clock dial, and ending at about 2 oclock onthat dial. When tube 13a is at the bottom, or at 6 oclock on the largerimaginary dial the weld fillet 14 extends on its small imaginary dialfrom 11 oclock to 5 oclock, and when the tube'is at 9 oclock on thelarger dial the fillet extends from 11 to 8 oclock. The reason for thislag of one hour by the weld 14 on the smaller imaginary clock dial withrespect to the position of the tube 13a on the larger imaginary clockdial will become apparent as the description of the invention proceeds.

As shown in FIG. 1, the tube sheet 12 and tubes 13 form part of aheatexchanger 15, which also includes a circular cylindrical shell =15ajoined to the edge of the tube sheet 12 and enclosing the tubes 13. Thewall 15a extends perpendicular to the plane of the paper in FIG. 1. Asprocket 15b surrounds and is secured to the shell 15a at a regionintermediate its ends and meshes with asprocket chain 16, which isdriven by a sprocket 17, driven by a source of power (not shown). Theheat exchanger 15 rotatably rests on four idler wheels, the front two ofwhich are shown at 18, and these are rotatably mounted on brackets 19attached to a table 19a.

A mobile frame 20 is provided, which comprises upper and lowerhorizontal beams 21 and 21a and vertical beams 21b. Near one end of beam21 a pair of closely spaced pulleys 22 and 23 is rotatably mounted. Nearthe other end of beam 21 a pair of widely spaced pulleys 24 and 25 isrotatably mounted. The spacing of pulleys 24 and 25 is equal to thedistance between rope holes 27 and 27a in a horizontal cross member 28of a harness 28a for the welding gun 10. Ropes 29 and 30 connect harnessbeam 28 to a counterweight 31, rope 29 being connected to the hole 27near one end of beam 28 and passing over pulleys 25 and 23, and rope 30being connected to the hole 27a near the other end of beam 28 andpassing over pulleys 24 and 22. Since the pulleys 24 and 25 are on thesame horizontal line and have the same spacing as the rope holes 27 and27a in cross member 28, the ropes 29 and 30 and counterweight 31 keepthe cross member 28 horizontal regardless of its position. Thus, thecross member 28 is horizontal, both in the full-line position and in thebroken-line position of FIG. 1.

The harness 28a has a main vertical member 32, which as shown in FIG. 1has its upper end secured to a mid region of the cross member 28, and,as shown on FIG. 2, has at its lower end a horizontal extension 33. Theextension 33 is attached by brackets 33:; to the grip it of the weldinggun 9. A split collar 34 which is joined by a peripheral weld 35 to thevertical member 32, receives a tubular fitting 3d of the welding gun 9,the tubular member extending through the vertical member 32. A clampingband 37, regulated by a bolt 38 and nut 39, permits the collar 34 to beclamped against the tubular member 36.

A knurled cap 4i), having an internal screw thread engaging an externalscrew thread on an extension 40a in a junction block 41, clamps a head41a of tubular fitting 36 against extension 49a and thus secures thejunction block to the tubular fitting. As shown in FIGS. 2 and 8, thetubular fitting 36, which is of brass, extends into a body 42 of the gun9 and carries a brass tubular insert 43 soldered to the fitting. Theinsert 43 receives a nylon tube 44 through which a consumable wireelectrode 45 moves. Shielding gas for the welding operation flowsthrough the tube 44 as well as through an annular cylindrical space 45abetween the insert 43 and the fitting 36. Shielding gas reaches thespace 45:; from the gun body 42 through openings 46 in the fitting 36.

The junction block 41, which is of brass, has two openings 4'7 whichcommunicate with the space 45a between fitting 36 and insert 43 andcarry the ends of two parallel spaced brass tubes 4-8 for shielding gas,which are secured to the block 4-1. The block 41 also has an opening 49in which is secured a brass tube 50 which is aligned with the tubularinsert 44 and has between the tubes 43 in parallel spaced relationshipthereto. A series of nylon tubular sections 51 fit in the tube 59 inend-to-end abutting relation with one another and the nylon tube Asshown on FIGS. 2 and 6, a sleeve 54- receives any of the tubes 13 to bewelded and has spaced constricted portions 55 fitting the tube forpositioning the gun g for a welding operation. The sleeve 54 has at oneend a collar 56, which is secured to the junction block 41 with aninsulator 5'7 therebetween, by means of screws 58, which are insulatedfrom the collar 56 and threaded into the block 4-1. As shown in FIGS. 2,3 and 6, two coolant tubes 59 are secured at one end by brazing, in thecollar 56 and communicate through L-shaped passages 68 in the block 41with lines 61, which are connected with the bottom of the block. Thecoolant tubes 59 are outside the sleeve 54 and are parallel with, andspaced from tubes 48 and Stl. The sleeve 54 is provided with anadjustable stop 62 which is engageable with the end of the tube 13 beingwelded. The stop 62, which lies within the sleeve 54, extends into alongitudinal stop 63 in the sleeve and is fixed thereto by a Washer 64engaging the outside of the sleeve and a screw 65 projecting through theWasher and threaded into the stop 62.

The sleeve 54 has a collar 66, which is generally like the collar 56 butat the opposite end of the sleeve. The collar 66 has openings receivingthe ends of coolant tubes 59, which are brazed to the collar. Screws 67,which extend insulated through collar 66 and a brass ring 68 and arethreaded into nozzle 6%, secure ring 68 to the collar 66 and the nozzle11 to the ring. The ring is insulated from the collar 66 and the nozzle11 by gaskets '70 and 71.

The shielding gas tubes 48 and the electrode tube 5%, which clear thecollars 56 and 66 of sleeve 54 at large external recesses in the collarsas indicated at 72 for the collar 56 in FIG. 6, have their ends brazedto the ring 6'8 at openings '73 and '74, respectively, as shown in FIG.3. As shown in FIGS. 2 and 5, the ring 68, which is of brass, has agenerally concave interior surface 75 into which four passages 75 extendfrom the openings 73 for the shielding gas tubes, two passages from eachopening 73.

The nozzle 69 is formed of two brass parts '77 and 78 brazed together.As shown in FTGS. 2, 3, and 4, the nozzle part '77 has openings '79which are aligned with opening 73 in the ring 68 and communicate with awide arcuate opening 89 in the nozzle part '78 through a wider arcuaterecess 31 in the end of nozzle part 78 facing part '77. The recess 81communicates with an opening 82 in the nozzle part '78.

A copper contact tube 83 for the wire electrode 45 is secured to thering 68 in electric contact therewith in the opening '74 by a set screw34 and abuts the end tubular section 51 so as to prevent the series ofsections 51 from being pulled or pushed out of the tube 50 by the wireelectrode 45 as it moves toward the tube sheet 12. The contact tube 83passes loosely through the openings 82 and st in the nozzle 11, withoutcontact with the walls of the openings, so that there is no electricalcontact between the contact tube and nozzle and there is space in theseopenings around the contact tube for shielding gas. As shown in FIG. 3,the ends of the electrode 45 and the contact tube 33 lie in a recess orpocket 85 formed in the end of the nozzle part 78.

The ring 63 has openings 86, shown in FIG. 5, and the nozzle part '77has openings 86a aligned therewith. Through these sets of openings, thecoolant tubes 59 communicate with an extensive arcuate coolant recess 37formed in the nozzle part '78, as shown in FIGS. 2 and 4. The recess 8'7is relatively shallow except at relatively deep end portions 38, whichare adjacent the opening 89, contact tube 83, and wire electrode 45 forthe sake of etlective cooling of the welding region.

Conduit 83 protrudes into the space 85 almost to the tip of nozzle part'78, and electrode 45 extends slightly beyond that to a point where itcan form a welding are between itself and the workpiece, which is tubesheet 12 and tube 13 at their common boundary.

FIG. 9 shows the product in the invention in more detail. Up until nowthe tubes 13 being welded have been referred to as if they were a unit,since to do otherwise would be to go into confusing detail. In FIG. 9,the numeral 13 designates the tube generally as before, and the numerals13b and 130 designate the inner and outer tubes which make up thecomposite type of tube that is customarily used in heat exchangershandling liquid metal for reasons of safety. However, for purposes ofthe present invention, the exact type of tube used is immaterial. Whatis important from the standpoint of the invention is the weld 14 whichjoins the tube sheet 12 to the tube 13, or more specifically, to itsouter part 13b. It will be observed that a cross section of the weldresembles a right triangle with equal legs and a hypotenuse that is onlyslightly concave. his is the strongest possible configuration for a weldof this kind, and it is something that can be achieved only by thepresent invention with any degree of reproducibility.

To carry out the invention the welding gun 9 is placed over one of thetubes 13 in the manner shown in FIG. 2, and the stop e2 is adjusted sothat the tip of the contact tube 83 comes to the proper distance fromthe boundary of the tube sheet 12 and the tube 13, or of the outer tube1312 if the tube is of the composite type. It will be noted that thetube 83 is bent downward slightly at its end; this causes the wireelectrode 45 to bend downward also so as to confront the boundaryexactly.

The shielding gas is then turned on for a time sufficient to purge thevicinity of the arc of air. Any shielding gas such as argon, helium andthe like may be used, but I have found that a mixture of 5% by volume ofoxygen and the balance argon gives unexpectedly good results. 60% ofthis mixture appears to be equivalent to of pure argon in shieldingpower for some reason that is not apparent. However, any method ofwelding is operable even in the absence of shielding gas, although whenthis is the case the presence of either oxides, flux particles or bothin the weld fillet necessarily weaken it to someextent.

If, as is preferable, a shielding gas is to be used, as soon as the airhas been purged from the site of the arc the welding current is turnedon and simultaneously the source of torque rotates the sprocket 17, andthis, in turn, rotates the entire heat exchanger on the idler wheels18by the action of drive chain 16 on the sprocket ,17.

The rotation of the heat exchanger 15 is for one full turn. The rate ofrotation necessarily depends on the materials being welded, the densityof the welding current and the length of the boundary being welded. Forstainless steel tubes of 1.438 inches outer diameter, or a boundaryabout 4.51 inches long, I have found that 15 seconds is to be preferredfor making the turn of 360 degrees.

The gun9 is then withdrawn and placed over another of the plurality totubes 13 and the processrepeated until all the tubes in the sheet havebeen sealed to it by welds. It should be noted that no matter how faroff center a tube may be, such as the tube 13a, from the axis of theheat exchanger 15, the method'will still work, since the mainharnessmember 32. carrying the welding gun 9 is made to move horizontally andvertically while remaining parallel to itself in all, because thepulleys 24 and 25 are horizontally related toone another and are spacedthe same amount as the rope holes 27 and 27a in the horizontal harnessmember 28, and the ropes 29 and 30 are kept tight by the counterweight31.

Since the vertical orientation of the main harness member 32 ispreserved even through the harness itself may be describing a circle oflarge radius, it is possible to orient the position of the electrode 45with respect to the vector of gravity by means of the cap 40. I havefound that theorientation of the electrode 45 has a marked effect on thequality of the weld fillet. For most cases the electrode-45 should beoriented from 30 to 60 degrees from the vertical in the directioncontrary to the direction of the rotation of the tube 13. It is to beunderstood that the tubes 13 must fit the perforations in the tube sheetquite snugly so that when the heat exchanger 15 rotates as a whole thereis no slippage, and thetubes accordingly rotate in the same direction asthe heat exchanger and through the same'number of degrees.

For example, when the tube 13 turns in the clockwise direction, theelectrode 45 should be from 30 to 60 to theleft, or at from about 10 to11 oclock, this less exact method of expressing degrees beingsufliciently accurate for purposes of carrying out the invention. Whenthe rotation is counter-clockwise the electrode 45 should be oriented tothe right, or from about 1 to 2 oclock.

, The foregoing explains the lag of one hour between the weld, and heatexchangers rotation above referred to in connection with FIG. -1.. Thetube may be in any po sition at the start, just so that the heatexchanger makes a full rotation during the welding. In FIG. 1 the placeof starting is arbitrarily set at 12 oclock, but it could bet set justas well at any other position without affecting the position of theelectrode 45 on its smaller imaginary clock dial. However, for purposesof making the drawing some starting position had to be selected, and the12 oclock position was selected because it is the easiest to explain,and furthermore, it is preferable to choose this starting position inthe practical operation of the invention. It is important that thewelding be carried out for one turn, no more, no less, and it is easiestto see that this is done by starting and stopping at the top position.The drawing also selected the preferred 11 oclock position as thestarting welding position on the smaller imaginary clock dial, and thisresults in a one hour lag, since when the tube 13 has advanced threehours to 3 oclock, the weld fillet will only have advanced to 2 oclock,it having started with a handicap of one hour. Again, if the electrodehad originally been oriented at 10 oclock on its dial, there would be atwo-hour lag instead of a one-hour lag,

the starting point for the tube, of course, being 12 oclock. In the caseof counterclockwise rotation, the electrode 45 would then start at from1 to 2 oclock and if the tube started at 12, the weld fillet would notshow a lag but a advancement if the tube. started at 12 oclock. I I

The amount of orientation of the electrode 45 away from the verticaldepends on a number of factors, such as the type of welding electrodeused, its composition, whether consumable or nonconsumable, thecomposition and thickness of the tubes 13 and tube sheets 12, the speedof rotation of the tubes 1, the voltage, current density, and the like.In general, I prefer a consumable electrode having no flux component, aflux not being necessary when shielding gas is used.

I find that I can quickly gauge the soundness of the fillets produced bymy invention by visual inspection. If the face of the fillet, or thehypotenuse of the triangle shown in FIG. 7,'is more than slightlyconcave, it is an indication that the tube 13 isbeing rotated too fast.If the fillet is convex it indicates that it is being rotated tooslowly. A lumpy, uneven fillet indicates that the electrode is orientedtoo close to the vertical, while a fillet showing unevenness due tohollowed-out places indicates that the orientation is too far from thevertical. The speed of rotation and the orientation should be adjustedto avoid those defects, and fortunately with my apparatus this is doneWithout undue difficulty since my apparatus tends generally to makeequilateral fillets even when not inperfect adjustment. This is incontrastto the previous methods where the tubes were held verticalduring the welding; in these the tendency was for the fillets to belonger along the tube sheet than along the tube due to gravity, and itwas difiicult to equalize its dimensions. In my method where the tubesheet is vertical and the tubes horizontal, it might'be expected thatthe vector of gravity would cause the fillets to have greater lengthalong the horizontal tubes than along the tube sheets for the samereason. However, with my offset electrode apparatus, with itsorientation of the electrode away from the verti cal as the tube doesthe rotating, the tendency of gravity to produce unequal dimensions inthe fillets is completely overcome. I

The significant thing about the present welding arrangement is that theparticular tube 13 being welded to the sheet 12 is rotated so as to movethe region just welded upwardly at a slight vertical angle (clockwisefrom 10 or 11 oclock) from the region about to be or now being Welded.The result is that the weld metal just laid down tends to move whetherfrom the consumable electrode 84 or other source of added material bygravity into the re: gion'nowor about to be welded, so that there isalways weld metal at the region where the electric arc is directed.Consequently, the tube 13 is protected from the direct action of the arcand is not so likely to be burned and perhaps perforated.

Example I A heat exchanger for exchanging the heat from a NaK alloycoolant from a nuclear reactor to water was fabricated according to theinvention. The heat exchanger was of the composite-tube type having twotube sheets at each end, or a total of four tube sheets in each heatexchanger. Each heat exchanger had 73 tubes running through the fourtube sheets. The tubes were of the composite type having an inner tubeof 1.065 inner diameter and 1.250" outer diameter, and an outer tube of1.250" inner diameter and 1.438" outer diameter. The outer tube wassomewhat shorter than the inner tube, and extended through the two innertube sheets but not through the outer ones. The material of the tubesand tube sheets was an alloy of 2 A Cr, 1% Mo, both by weight, and thebalance Fe.

In sealing the tubes to the tube sheets an apparatus similar to that ofFIG. 2 was used. The electrode 27 was a consumable wire, 0.035" indiameter, of 1 A Cr, /2 Mo, both by weight, and the balance Fe.

a The method of the invention was carried out under the followingconditions:

Voltage of welding arc 28 volts.

Amperes of welding current 250 amperes.

Cover gas 25 cu. ft. Ar/hour. Feed of electrode wire 660 inches perminute.

Time of rotation of workpiece 15 seconds. Direction of rotation ofworkpiece Clockwise. Orientation of electrode 11 oclock.

Example II An identical heat exchanger was fabricated under the sameconditions except that the cover gas was a mixture of oxygen and thebalance argon which was fed at the rate of 15 cubic feet per hour.Results were equally good.

Examples IIIX Eight more, or a total of ten, heat exchangers werefabricated according to the methods of the preceding examples. Sinceeach had 73 tubes and each tube passed through four sheets, 292 weldswere required in each heat exchanger. For the ten heat exchangers 2920welds were required. Of these 2920 welds only 4 showed signs of leakage,or 0.137%. This is a far lower percentage than has ever been achieved inany other method of tube sealing to tube sheets.

It will be understood that this invention is not to be limited to thedetails given herein, but that it may be modified within the scope ofthe appended claims,

What is claimed is:

1. An apparatus for welding a tube to a tube sheet of a heat exchanger,the tube being held by the tube sheet with sufficient snugness toprevent slippage, the apparatus comprising a sleeve exteriorlyengageable with the tube, a nozzle attached to one end of the sleeve soas to be adjacent the tube sheet, means for feeding a consumable weldingelectrode along the sleeve and through the nozzle toward the boundarybetween the tube and the tube sheet, a harness attached to the end ofthe tube away from the nozzle, a pair of spaced pulleys located at thesame level, a pair of ropes connected at one end to regions of theharness having the same spacing from one another as the pulleys, theropes extending over the pulleys, a common counterweight attached toeach of the ropes at the end remote from the harness, means for causinga Welding current to flow from the electrode to the boundary between thetube and the tube sheet, and means for rotating the tube and tube sheetabout an axis parallel with the tube.

2. An apparatus for welding a tube to a tube sheet of a heat exchangerhaving a cylindrical surface, the tube being held by the tube sheet withsufficient snugness to prevent slippage, the apparatus comprising asleeve exteriorly engageable with the tube, a nozzle attached to one endof the sleeve so as to be adjacent the tube sheet, means for feeding aconsumable Welding electrode along the sleeve 14.1 and through thenozzle toward the boundary between the tube and the tube sheet, aharness attached to the end of the tube away from the nozzle, a pair ofspaced pulleys located at the same level, a pair of ropes connected atone end to regions of the harness having the same spacing from oneanother as the pulleys, the ropes extending over the pulleys, a commoncounterweight attached to each of the ropes at the end remote from theharness, means for causing a welding current to flow from the electrodeto the boundary between the tube and the tube sheet, a plurality ofwheels engageable with the cylindrical surface of the heat exchanger forrotatably supporting the same, and means adapted to be wrapped aroundsaid cylindrical surface for rotating the heat exchanger, and means forrotating the tube and tube sheet about an axis parallel with the tube.

3. A method of sealing the interface between a tube sheet in a verticalposition and a horizontal tube fitting in an opening through the sheet,the said opening being located a distance from the center of said sheet,comprising rotating the sheet 360 degrees about its center while causingthe tube horizontally to follow the said opening about the said centerwithout exerting sufiicient torsion on the tube to cause it to slipwithin the opening, simultaneously following the interface between thetube and the tube sheet with a movable consumable wire electrode whilepreventing rotation of the electrode and holding the electrode parallelto the tube and displaced from the top of the tube about 30 to measuredin a direction opposed to the rotation of the tube, and simultaneouslymaintaining a continuous welding are between the electrode and theinterface during the entire 360-degree rotation of the tube and the tubesheet.

4. An apparatus for welding the interface between an opening in a tubesheet and a tube fitting through the opening with sufiicient snugness toprevent slippage, the said opening being located a distance from thecenter of the tube sheet, comprising means for causing rotation of thetube sheet 360 degrees about its center, means for causing the tubehorizontally to follow the opening about the said center withoutexerting sufficient torsion on the tube to cause it to slip within theopening, a welding electrode, means for causing the electrode to followthe interface about the said center, means for maintaining constant theorientation of the electrode with respect to the vector of gravity,means for causing a welding current to flow between the electrode andthe interface simultaneously with the rotation of the tube sheet.

References Cited in the file of this patent UNITED STATES PATENTS

3. A METHOD OF SEALING THE INTERFACE BETWEEN A TUBE SHEET IN A VERTICAL POSITION AND A HORIZONTAL TUBE FITTING IN AN OPENING THROUGH THE SHEET, THE SAID OPENING BEING LOCATED A DISTANCE FROM THE CENTER OF SAID SHEET, COMPRISING ROTATING THE SHEET 360 DEGREES ABOUT ITS CENTER WHILE CAUSING THE TUBE HORIZONTALLY TO FOLLOW THE SAID OPENING ABOUT THE SAID CENTER WITHOUT EXERTING SUFFICIENT TORSION ON THE TUBE TO CAUSE IT TO SLIP WITHIN THE OPENING, SIMULTANEOUSLY FOLLOWING THE INTERFACE BETWEEN THE TUBE AND THE TUBE SHEET WITH A MOVABLE CONSUMABLE WIRE ELECTRODE WHILE PREVENTING ROTATION OF THE ELECTRODE AND HOLDING THE ELECTRODE PARALLEL TO THE TUBE AND DISPLACED FROM THE TOP OF THE TUBE ABOUT 30* TO 60* MEASURED IN A DIRECTION OPPOSED TO THE ROTATION OF THE TUBE, AND SIMULTANEOUSLY MAINTAINING A CONTINUOUS WELDING ARC BETWEEN THE ELECTRODE AND THE INTERFACE DURING THE ENTIRE 360-DEGREE ROTATION OF THE TUBE AND THE TUBE SHEET. 